1. Field of the Invention
The present invention relates to a speaker system having a plurality of double-voice-coil speaker units, a closed type cabinet, and an impedance compensating circuit for making the resistance of an input impedance constant.
2. Description of the Related Art
FIG. 5 shows a structural example of a conventional speaker disclosed by Watkins (W. H. Watkins, "New Loudspeaker with Extended Bass," Audio, Dec. 1974, pp.38-46) which is designed to improve a sound radiation efficiency in a low sound region by using double voice coils. In FIG. 5, reference numeral 100 denotes a signal input terminal to a speaker; 10, an impedance compensating circuit made up of a series circuit consisting of an inductance L and a capacitance C for making the resistance of an electric impedance of the speaker constant; 20, a double-voice-coil speaker unit; 21, a first voice coil; 22, a second voice coil; and 25, a bobbin for fixing those voice coils 21 and 22.
In general, in the case where there is no impedance compensating circuit 10, that is, in the case where only the first voice coil 21 is used, the electric impedance of the speaker unit 20 forms a parallel resonance circuit so that a characteristic peak occurs in the vicinity of a minimum resonance frequency f.sub.0 of the speaker unit as shown in FIG. 6. For that reason, a current that flows in the first voice coil 21 is reduced in the vicinity of the resonance frequency f.sub.0, to thereby reduce the efficiency of a sound radiated from the speaker.
Therefore, in FIG. 5, an impedance compensating circuit 10 made up of a series circuit consisting of an inductance L and a capacitance C is added in series to the second voice coil 22 in such a manner that a series resonance circuit having a resistance provided by the second voice coil 22 is connected in parallel with the first voice coil 21, thereby making the electric impedance constant at all times (making the resistance constant) to improve the sound radiation efficiency.
FIG. 7 shows a change in sound pressure level in a low sound region due to the presence/absence of the impedance compensating circuit 10.
As is apparent from FIG. 7, a rise of the sound pressure level, that is, a rise of the sound radiation efficiency is found in the vicinity of the minimum resonance frequency f.sub.0 with making the resistance of the impedance constant using the impedance compensating circuit 10. The effect reaches to the extent that the sound pressure level reaches several dB, depending on the conditions.
FIG. 8 shows a system extended from the conventional speaker structure shown in FIG. 5, in which a plurality of double-voice-coil units are used.
In FIG. 8, reference numeral 10-1, . . . , 10-i, . . . , 10-n denote impedance compensating circuits; 20-1, . . . , 20-i, . . . , 20-n, double-voice-coil speaker units; 21-1, . . . , 21-i, . . . , 21-n, first voice coils; 22-1, . . . ,22-i, . . . , 22-n, second voice coils; and 25-1, . . . , 25-i, . . . ,25-n, bobbins for fixing the voice coils. The same or like parts as those in FIG. 5 are indicated by the identical references.
By the way, in order to realize the low resistance in the conventional speaker structure, there is required one impedance compensating circuit for each double-voice-coil unit. To satisfy the above requirement, an impedance compensating circuit corresponding to each speaker unit is required in FIG. 8.
As shown in FIG. 8, in the case where a plurality of speaker units are used, an impedance compensating circuit is required in correspondence with each unit in the conventional speaker structure. Therefore, in the speaker system using a plurality of units, there are required a plurality of impedance compensating circuits. As a result, such a speaker unit is generally expensive, and an interior of the cabinet is occupied by those plural impedance compensating circuits from the spacial viewpoint, resulting in a problem that a volume necessary for low-sound reproduction cannot sufficiently be ensured.